This term is usually associated with online games. The term is used in the event a player describes a situation in which the outcome was unbelievable, unlikely, or amazing. Player(A) then describes the event to his friend, player(B), who was not there to witness the event, which to him sounds unbelievable. In which case, player(B)in disbelief says, "Screenshots or it didn't happen."

Player(A): LOLWTFOMG!11!!cos(0)1!!!! HAHAHAHAH, I just one shotted a paladin through his nub shield.

Player(B): Screenshots or it didn't happen!

Player(A): HAHAHLOLOMGWTF!!1!!!cos(0)!1!! I just shot my tube from The Hotel across the map and killed player(C) at the Gatehouse on Karkand in BF2 without knowing where he was.